Core–shell structures in pressureless-sintered undoped Pb(Fe2/3W1/3)O3 ceramics Original Research Article

Core–shell structures have been found by both scanning and transmission electron microscopy in undoped Pb(Fe1/3W2/3)O3 ceramics when a solid-state-reacted powder of stoichiometric composition was sintered at 800°C. The presence of superlattice reflections of the {1/2 1/2 1/2}-type, often termed F-spots, indicates that the existence of nanoscale ordered domains of 2–5 nm size in both core and shell is due to chemical ordering of the B-site cations with alternating Fe3+ and W6+ along the <111> direction. The ceramic is densified by the assistance of a low-temperature liquid eutectic at ∼690°C. A formation mechanism of core–shell structures in Pb(Fe1/3W2/3)O3 ceramic is proposed. It can be described by solution-reprecipitation and phase transformation subsequently to perovskite Pb(Fe1/3W2/3)O3, involving at least two intermediate phases which are formed at temperatures below and above the low-temperature liquid eutectic point and related to pyrochlore (Pb2FeWO6.5). The cores are characterised by “black dots” and contain relatively Fe-excess. The shells exhibiting featureless contrast are free of “black dots” but contain W-excess relative to the cores. Long term sintering at 800°C has facilitated the inward diffusion of W6+ to the grain interior when the distribution of the B-site cations is improved. As a result, the shell increases its size at the expense of the core.